1. Field of Invention
The embodiments of present invention relate to a pixel structure. More particularly, the embodiment relate to a pixel structure and the driving method thereof in a light emitting diode display panel.
2. Description of Related Art
With the development of the display technology in recent years, the flat-panel display is widely utilized in daily life. Because active matrix OLED (AMOLED) possesses the characteristics of high quality, high contrast, rapid response, it becomes popular for consumers.
For a conventional AMOLED, each of the pixels includes two transistors (writing transistor and driving transistor), a pixel capacitor and an organic light emitting diode. When the writing transistor of the pixel structure is conducted by the scanning signal, the data signal is read and temporally stored in the pixel capacitor. At this moment, the driving current of the light emitting diode from the driving transistor may be calculated by the formula as below:
  I  =            1      2        ⁢                  β        ⁡                  (                      Vgs            -            Vth                    )                    2      
In the formula above, I represents the driving current; β represents a constant number; Vgs represents the differential potential of the source/drain electrode in the driving transistor; and Vth represents the threshold voltage of the driving transistor.
Owing to the manufacturing variation of different pixels, the transistors thereof may have different threshold voltages. Accordingly, the driving currents of different pixels may be varied so that the brightness of the organic light emitting diode is not uniform.
Besides, after an operation period of the organic light emitting diode, the electrical characteristics of the OLED may be changed easily. With different emitting status for every pixels on the panel (for example, high brightness, low brightness, long emitting period, alternative emitting and so on), the decay level of OLED's characteristics is not uniform so that it may result in non-uniform brightness.
Furthermore, with the size of the panel enlarging, it needs longer signal wires to transmit power signal (for example, system voltage OVDD) to the pixels. The longer the signal wire is, the higher the wire resistance is. Accordingly, the current transmitted to the pixels is decreased so that it may result in non-uniform brightness.